A combination of dense felt on the back of the driver and toilet roll in the enclosure and an electrical tape reflex dot on the front. Also sealing the earcup as much as realistically possible to create a fixed airspace. I played around a lot with different amounts of loo roll. The distance that the driver is mounted from the ear in the housing also matters. The problem isn't eliminated, just tamed to a level that I find personally acceptable. Theres still a sheen, its just not grating anymore and that lowest end really does have some physical thump to it without the typical moving coil blunt smack, its like the whole earcup carries the energy rather than having the energy thrown down your ear.
The sound also changed notably depending on how it was amped, much drier from the B-Tech box run of the speaker taps, warmer and more mids but fuzzier out of the headphone socket on the front of the NAD. More limited dynamics and frequency extension out of an ipod or laptop.